Compiler error matching variable with enum with all unit variants

[xunilrj]

The following code:

enum OnlyUnits {
    A: ()
}

match some_value {
    OnlyUnits::A(x) => {}
}

generates a compiler error:

 error
            --> /home/xunilrj/github/sway/test/src/e2e_vm_tests/test_programs/should_pass/language/logging/src/main.sw:59:11
             |
          57 | 
          58 | 
          59 |     match F::A {
             |           ^^^^ Internal compiler error: Failed to get variant type from enum in `unsigned downcast`.
          Please file an issue on the repository and include the code that triggered this error.
          60 |         F::A(x) => {}
          61 |     }
             |
          ____

The problem seems to be a optimization that we do when an enum only has unit variants (its type is { u64 }, only the discriminant) and the fact that the match "tries" to access the variant data (the second element of the struct).

[ironcev]

Actually, the compilation shouldn't reach the IR generation stage because of the type mismatch. The issue is that we don't check variant type in the pattern against the declared variant type, if the pattern contains variables (as x in the above example). See #6286.